1. Field of the Invention
The invention relates generally to the field of analysis of various physical properties of porous materials, including Earth formations. More specifically, the invention relates to methods for estimating pore structures of porous materials from selected petrophysical information, and using the estimated pore structures to infer other properties of the porous materials.
2. Background Art
Many materials, including Earth rock formations, are porous, meaning that some of the volume occupied by the rock formations has void spaces therein. Many porous materials are also permeable to the flow of fluids. Porous, permeable Earth formations, in particular, are widely explored for the presence of useful materials, such as petroleum. Exploration techniques known in the art for evaluating porous, permeable Earth formations beneath the Earth's surface include seismic surveying, flow testing and well logging, among others. Among the purposes for Such exploration techniques, particularly as applied to petroleum-bearing formations (“reservoirs”), is to assist in the prediction of fluid production from the Earth formations as the petroleum is withdrawn from subsurface reservoirs. Predictions of fluid production are useful in estimating how many wellbores may be needed to extract the maximum amount of producible petroleum from a particular reservoir, and the rates at which the petroleum will be produced, as well as rates at which connate fluids (typically water) will be produced from the reservoir. Such predictions are important in determining the economic value of a particular reservoir.
As is known in the art, fluid production is related to factors including the fraction of the pore space occupied by the petroleum and occupied by connate water (which typically occupies the remainder of the pore space) and the relative mobilities of the petroleum and the connate water. Relative mobility of a particular fluid is related to the fraction of the pore space occupied by the particular fluid, the viscosity of the particular fluid, and the capillary pressure behavior of the particular fluid within any particular porous material or Earth formation.
Capillary pressure behavior of a particular fluid is related to a great extent to the geometric structure of the pore space (“pore structure”) in a porous material or Earth formation. Pore structure is related to factors including sizes, size distribution, shapes and chemical composition of the mineral grains (“matrix”) forming the particular porous material or Earth formation. Methods known in the art for determining pore structure include micrographic (including image) analysis of “thin sections” of an Earth formation. A thin section is a representative sample of the formation cut or ground to a thickness typically less than 0.5 mm such that visible light will penetrate the sample for image analysis. It is also known in the art to measure capillary pressure behavior of particular porous materials or Earth formations by measurements of mercury injection pressure on actual samples (“cores”) of the particular formation. It is also known in the art to correlate various classes of pore structures to capillary pressure behavior.
Irrespective of the method used to determine the capillary pressure behavior of particular porous materials, accurate determination of capillary pressure behavior using methods known in the art requires analysis of physical samples of the particular formations, whether the samples are thin sections, or core samples or the like. Obtaining samples of each particular formation which includes petroleum reservoirs is difficult and expensive. Further, it is well known in the art that physical properties of subsurface Earth formations vary, sometimes greatly, within a geographic volume (product of the areal extent and thickness) coextensive with a petroleum reservoir. Therefore, accurate prediction of capillary pressure behavior, and corresponding fluid production, from a particular reservoir using techniques known in the art would require large numbers of samples of the particular formation for analysis of the geometric distribution of capillary pressure behavior.
The previously described exploration techniques, namely seismic surveying and well logging, provide relatively rapid, volumetrically extensive estimates of the structure and composition of subsurface Earth formations. However, methods for estimating capillary pressure behavior from such exploration techniques have proven to be impractical.
One way to estimate capillary pressure behavior from seismic date and well log data known in the art is to model the pore structure of the subsurface Earth formations from other measurements of properties of the formation. One such modeling technique is described in, Doyen, P. E., Crack geometry of igneous rocks: A maximum entropy inversion of elastic and transport properties, J. Geophysical Research, v. 92, n. B8, pp. 8169-8181, (1987). Conceptually, the method disclosed in the Doyen reference includes generating an initial model of the Earth formation as a homogeneously distributed network of interconnected tubes. The tubes have an average length, average diameter and average aspect ratio. The tube variables are adjusted such that selected predicted physical properties of the particular rock most closely match corresponding measured properties of the formation.
The method disclosed in the Doyen reference has proven to be impractical when applied to typical petroleum-bearing formations. In particular, the Doyen method has not provided accurate predictions of a property known as the “formation factor” (a ratio of the electrical resistivity of the particular formation with respect to the resistivity of fluid filling the pore spaces).
What is needed is a method for accurately estimating pore structure of formations from data such as seismic surveying and well logs, such that capillary fluid pressure behavior and other pore structure related properties of porous materials, particularly subsurface Earth formations, may be accurately estimated using such data.